Something that would allow a user to safely arm the fire control circuit, and maybe provide some DRAMATIC sound effects.

Maybe something like the sound of a cheap camera charging it's flash. It would be fun, and this would also alert by-standers, and Tess to be ready for retrieval.

I am thinking, For Arming Fire Control

From Idle

Quick push and release

Up Ramping Tone, followed by push, 3 second hold and Release

Med Tone, One second delay, flashing Blue LED

High Tone, Fire on next Release if within 3 seconds, flashing Red LED, Else ABORT

Down Ramping Tone

Return to Idle

To Change Valve Open Time

From Idle

Push, hold for 4 seconds

Report CW "P" (for Program Mode)

Quick push and release, with "boo-bonk" sound report for down

Two Quick push and release, with "boo-bink" sound report for up

Repeat two above, unless timeout

Report via CW "R" sound

Return to Idle

To Reset (ie reboot)

From Idle

Push and hold for 6 seconds

Report via CW "R" sound

Return to Idle

This style of human interface is used by the K1EL CW Keyer, which I find very usable. To provide tones and switch monitoring at the same time may require interrupt programming. A low idle power processor would avoid the need for a power switch.

The Valve Open Time would avoid un-necessarily expelling air after the Tennis Ball has left the barrel, which would be useful (but maybe a little dangerous when reloading?).

This would take very little SMT hardware and could be made on a very small PCB:

A user interface push button switch

Processor

A Small Piezo Speaker

Maybe an LED or two

A FET and/or Relay to drive the Air Valve

A few Headers for the 9 Volt Battery, Valve Connection and maybe a JTAG

The circuit board can be created quickly with the Toner Transfer Method as used by my other project.

Now all I need is to find a low drain power processor, and re-learn my programming skills,

Or, maybe someone would want to help?

UPDATE
Experiments today shows that I can save about 1/2 of the air charge by limiting the air valve open time to just that which is needed to shoot the ball from the barrel. But, this also may imply that there are some restrictions within the system; maybe the Valve, the bottle necks or the pipe size.